The present invention relates to gas sensors, e.g., lambda probes, including a body formed as a sintered ceramic laminate and a reference air duct situated therein within a layer of the laminate, an electrical resistance heater provided on its one side and an electrode configuration provided on its other side, the electrode configuration including at least one reference electrode that:is arranged on the inside of a boundary wall of the reference air duct and is permeable for gases at least regionally and that includes a Nernst electrode that is acted upon by gases to be sensed, the Nernst electrode is also at least regionally permeable for gases and separate from the reference electrode by a solid electrolyte layer that is conductive and permeable for ions, e.g., oxygen ions, and the electrodes connected to printed circuit traces that essentially extend in the direction of the reference air duct.
Exhaust systems of modern internal combustion engines, particularly for motor vehicles, are regularly provided with catalytic converters for converting harmful exhaust gases into harmless reaction products. In order for the catalytic converters to function well, it is necessary to feed air and fuel to the engine in a predefined proportion. The engine controls provided for this purpose are connected on their input side to a so-called lambda probe the signals of which represent the composition of the exhaust gas and, thus, enable the engine control to adjust the ratio of fuel and combustion air in a manner optimal for the catalytic converter.
Two configurations are conventional in this connection.
In the one configuration, stoichiometric combustion is targeted, i.e., the oxygen quantity in the combustion air corresponds exactly to the oxygen requirement for complete combustion of the supplied fuel. Therefore, the engine is operated using neither an excess of oxygen (xcex greater than 1) nor using a deficiency of oxygen (xcex greater than 1). This operating method may, therefore, by characterized by xcex=1.
When sensing exhaust gas,: narrow-band lambda probes where the Nernst electrode is acted upon by the exhaust gas as directly as possible are sufficient for this stoichiometric combustion.
In this instance, the effect is used by the engine control that an electrical voltage able to be tapped off between the reference electrode and the Nernst electrode and generated by diffusion of oxygen ions significantly changes its value in the vicinity of xcex=1, and a signal is consequently available that clearly displays a deviation from the desired operating mode using stoichiometric combustion in the direction of an operating mode having an oxygen deficiency as well as in the direction of an operating mode having an oxygen excess.
Such sensors are described in German Published Patent Application No. 44 01 749, for example.
In the other configuration, the objective is predominant operation of the internal combustion engine with an oxygen excess during combustion since the fuel consumption is able to be noticeably reduced as a result. However, during combustion using an oxygen excess, harmful nitrogen oxides are produced that may only be absorbed for a limited time by so-called adsorption catalysts in the exhaust branch of the motor vehicle. In each case prior to exhausting the absorption capacity of the adsorption catalysts, the engine operation must be switched over briefly to combustion with an oxygen deficiency in order for the incompletely combusted fuel components now entering the exhaust branch to be able to reduce the nitrogen oxides previously stored in the catalytic converter to nitrogen. In this instance, the engine control, i.e., the internal combustion engine, must be constantly switched at intervals between an operating mode that is predominant with respect to time and in which the values of xcex are greater than 1 and a relatively brief operating mode in which the values of xcex are less than 1.
Broadband lambda probes are necessary for such an intermittent operating mode having drastically changing values of xcex.
In the case of such lambda probes, the Nernst electrode is arranged at a separate chamber that communicates with the exhaust-gas stream via a diffusion path arranged in the body of the probe. Arranged within this chamber is also an internal pump electrode that may be electrically connected to the Nernst electrode and also cooperates through a solid electrolyte layer with an external pump electrode that is exposed to the exhaust-gas stream as directly as possible. If an external electrical voltage is applied between the two pump electrodes, which are both configured to be permeable for gases at least regionally, an oxygen ion current the direction of which depends on the polarity of the applied voltage and the intensity if which is determined by the electrical voltage difference as well as by the difference in the oxygen concentration at the pump electrodes is generated between the pump electrodes. This oxygen ion current accordingly controls the diffusion current of the exhaust gases in the diffusion chamber. The external electrical voltage between the pump electrodes and the electrical current occurring between the pump electrodes due to the oxygen ion current are adjusted by a controller so that an electrical voltage having a predefined setpoint value is always maintained between the reference electrode and the Nernst electrode. As such, the polarity and intensity of the electrical current occurring between the pump electrodes are a signal that correlates to the composition of the exhaust gases and, thus, to the xcex values.
Such probes are described in German Published Patent Application No. 37 44 206, for example.
Aging processes such as pollution change the properties of the aforementioned gas sensors.
In accordance with the present invention, it is provided that in a top view of the layer planes of the laminate, the printed circuit traces corresponding to the electrodes are arranged at least partially next to the reference air duct.
This may be applicable for the printed circuit traces of the pump electrodes.
The present invention is based on using the pressing pressure exerted during and/or prior to sintering the probe body to the laminate for compressing the composite structure of the printed circuit traces and in this connection to effectively increase the pressure forces exerted on the printed circuit traces by arranging the printed circuit traces in the laminate without being covered by hollow spaces as viewed from above. Consequently, a smaller electrical resistance of the printed circuit traces as well as a higher durability of the printed circuit traces with respect to aging effects is achieved with the result that the change in the electrical properties of the probe in the case of increasing age are significantly reduced.
In addition or alternatively, further measures may be provided. For example, the internal and/or external pump electrode may have a surface that is larger than the base plan of the gas chamber arranged between the Nernst electrode and the internal pump electrode.
The pump electrodes on a region diametrically opposed to the corresponding printed circuit trace, i.e., in the direction of the top end of the probe body, may extend beyond the base plan of the gas chamber.
Furthermore, it may be advantageous for constant electrical properties when the printed circuit traces have a comparably large layer thickness. For this purpose, the printed circuit traces may be produced using printing technology with relatively wide-meshed screens (e.g., screens including a 250 mesh). Moreover, the printed circuit traces may also be printed as a double-layer.
Finally, the pressure material for the printed circuit traces may have a high proportion of electrically conductive particles, e.g., based on platinum.
Example embodiments of the lambda probe according to the present invention are explained in greater detail below and are illustrated in the drawings.